The Strata And Their Geological Significance In The Canglaila Area Of Qiangtang Basin, North Tibet

The present paper deals with the strata and their geological significance in the Canglaila Area of Qiangtang Basin, North Tibet, and consists mainly of the following 5 parts: 1. The study area may be divided into three stratigraphic subprovinces. They are the Tanggula-Changdu Subprovince, belonging to Northern Qiangtang-Changdu-Simao Province of South China Realm, the Suoxian-Zuogong Subprovince included in Qiangnan-Baoshan Province of Yunnan-Tibet Realm, and the Eastern Qiaze Subprovince belonging to Bangonghu-Nujiang Province of Yunnan-Tibet Realm. The stratigraphic succession, their regional distribution and correlation of the three stratigraphic subprovinces are studied. 2. The pre-Ordovician "Youxi Lithodeme", a metamorphic series, is discovered in the area for the first time. Its degree of metamorphism is about high greenschist-amphibolie facies. The original rock of the "Youxi Lithodeme"is considered to be fine clastic rock intercalated with intermediate-acidic, intermediate, intermediate-basic pyroclastic rock and lava. 3. The Upper Triassic Dongdacun Formation, "Jiapila Formation"and "Bolila Formation"in the area may be correlative with the deposits in Taniantawong Mountain, western Yunnan and Changdu district, eastern Tibet. The Dongdacun Formation and the "Jiapila Formation"constitute a transgeressive sequence as a whole. The Dongdacun Formation is considered to consist in ascending order of debris flow deposits in alluvial fan, barrier-free clastic littoral deposits and tidal-flat deposits in shallow-water carbonate platform. The "Jiapila Formation"seems to form under the tidal-flats in carbonate platform, barrier bar and offshore environments during a xerothermal period,and earthquake events are also recorded in the formation. Three type of sedimentary sequences: earthquake-tsunami-background, earthquake-background, and tsunami-background depositional sequences are founded in the "Bolila Formation". 4. The newly named Chawula Formation consists of fine clastic rocks and limestones intercalated with coarse grain feldspathic sandstones, breccia mudstone and white fibrous gypsum and yields bivalves, brachiopods and starfishes. The bivalve Burmirhynchia-Holcothyris Assemblage is Batongian in age. The Chawula Formation is in angular unconformable contact with the underlying the pre-Ordovicion "Youxi Lithodeme", and is covered by the Niubao Formation of Paleocene-Eocene of age. 5. Ten main types of corbonate microfacies are distinguished in the Middle Jurassic Buqu Formation. Four sedimentary cycles may be recognized in the formation, which constitute a transgressive sequence. The formation mainly forms in 3 sedimentary environments: inner ramp, middle ramp and deep ramp. In the Buqu Formation asphaltum veins develope well and suffer successively from 3 main processes: liquid asphaltum filling-crushing-calcite cementation. It is considered that the Buqu Formation in Gongri area is not only a better oil-gas reservoir with the capability of autogeny and autodeposit, but also a prospective area of oil exploration.